Haven't properly gotten into Durge yet but bare with me:

from what we've seen, Durge's opinion of Orin wasn't too great; complaining that she is playing with her pray and the aftermath of them as with toys, taking her damn time and not really understanding the point of their Lord Bhaal - it is not the spectacle of mxrder but the act itself that is valued - destruction of all life - and in his name.

So imagine the rage, envy and animosity Orin had felt when her (in my case) 'big sister' - the favourite in the family who starts to get an even more important role in the grand scheme - starts to get entangled with Gortash and his manipulative ways of domination - and not destruction how Bhaal wills it.

As she herself says: '…the little lordling has been whispering in your ears? He always knew how to tumble and twist your mind matter, leave you knotted in his cords.'

Now, ignoring how absolutely deranged that line makes me (lmao) I can fully understand Orin, no matter how messed up she is: besides being the preferred spawn, Durge would also be getting away with meandering or straying from the course of their goal as well- no wonder she decides that she had enough of it and if she can't make herself the favourite, she can make herself the only…

How that pans out for her depends on the Durge, but can't say all too well hah.